In Devillers S.: “Bitstream Syntax Definition Language: An Input to MPEG-21 Content Representation,” there is a description of a method via which adaptation operations can be performed for given media streams on the basis of a language called BSDL (Bitstream Syntax Description Language) for the description of data streams. For this purpose transformation descriptions are used which can be written, for example, in the language XSL (Extensible Stylesheet Language). XSL transformations are explained in “XSL Transformations (XSLT)” Version 1.0, W3C Recommendation published Nov. 16, 1999. The transformation descriptions are used in order to transform an XML document conforming to a media stream-specific BSDL embodiment (i.e., a schema conforming to the “XML Schema Language” W3C Recommendation published May 2, 2001), or corresponding to a DTD (Document Type Definition), into an adapted XML document. Such document is then used, in turn, to generate a transformed data stream from a first data stream with the aid of the BSDL embodiment.
This process is explained in greater detail with reference to FIG. 1. A first data stream a_1 in the form of a binary bitstream is forwarded to a first bitstream parser BP_1. Using a BSDL schema BSDL_a, the bitstream parser BP_1 generates, from the first data stream a_1, a first data stream description BSD_1/1 for the first data stream in the form of an XML document. Using an XSLT style sheet XSL, an XSL processor XSLP then generates from the first data stream description BSD_1/1 for the first data stream a second data stream description BSD_1/2 for the first data stream. Finally, a second bitstream parser BP_2 transforms the first data stream a_1 using the BSDL schema BSDL_a and the second data stream description BSD_1/2 for the first data stream into the second data stream a_2.
This method can be used, for example, to adapt an instance of a media stream residing on a media server, such as a video stream, to the requirements of a requesting client such that, after the adaptation, the image format of the video stream, for example, corresponds to the screen resolution present on the terminal device or, for example, the preferred bit rate on the transmission path from the server to the client is used to optimal effect.
An example of a part of a first data stream description BSD_1/1 in the form of an extract from an XML document is shown in FIG. 2. In this example the data stream is described by the element “Bitstream”. The “Bitstream” element contains further elements which are specific to the described bitstream. Thus, an MPEG 4 bitstream is described in this example. The “Bitstream” element, therefore, contains further elements such as “I_VOP,” “P_VOP” or “B_VOP.” The contents of these bitstream-specific elements include a character string such as, for example, “akiyo.mpg4#18-4658,” which represents a reference to a bitstream. To that end, the character string is composed of a reference to the data stream (character string preceding the separator “#,” (for example, akiyo.mpg4) and a reference to the data stream subdomain classified by the element in the form of a start and end address in the data stream (e.g., 18 and 4658).
The data stream description presented has disadvantages, however. Thus, it is not possible, within the description BSDL_1/1, to add a further marking to the elements in addition to their name; for example, with regard to their importance for the overall presentation of the media stream.
The invention is, therefore, directed toward a new, improved description language for data streams which avoids the disadvantages described.